1. Field of the Invention
This invention relates to the field of controlling the fluid flow rate for hydraulic pumps, especially for automotive power steering pumps, and more specifically to the area of eliminating cavitation that results from pumping high viscosity fluid at extremely low temperatures.
2. Description of the Prior Art
At very low temperatures extending to near −40° F., the viscosity or resistance to flow of fluid used in automotive power steering systems increases by about 8000 times as compared to its viscosity at 275° F. At such low temperatures, the fluid flows like thick, heavy syrup.
Conventionally, power steering systems have a reservoir located remotely, or at least separate, from the hydraulic pump that pressurizes the system. The reservoir is usually placed in a relatively uncongested region of the engine compartment in comparison to the region surrounding the pump and drive belt sheave, by which the pump is driven from an engine. A pressure drop of 5-7 psi normally occurs at very low temperatures in a tube connecting the reservoir to the pump inlet. Another pressure drop of about the same magnitude is present within the pump between its inlet and the pumping chamber. These pressure drops result in an extremely low pressure, about 1 psi. in the supply chamber at very low temperatures.
When the engine is started in severely cold weather conditions, the pump speed immediately rises. However at such temperatures, the viscosity of the fluid is too high to permit sufficient flow of fluid from the reservoir to enter and fill the pumping chamber. This lack of fluid in the chamber cavitates the pump. It also causes an offensive high frequency noise that typically lasts for several seconds as fluid pressure in the steering assist valve supplied from the pump cycles rapidly between zero pressure to approximately 100 psi when some fluid does enter the supply chamber. The cyclic nature of the pressure variation is a consequence of successive short periods of sluggish flow through the pump, when a pumping chamber is at least partially filled with fluid, alternated by a short period when the pumping chambers are substantially fully vacant.
The resultant noise is objectionable and evidences a brief period during which the system or load is only partially pressurized. As flow rate increases following the cold start, fluid temperature rises rapidly to a temperature where pump cavitation ceases, the system becomes fully pressurized, noise disappears, and all other functions are normal.
To overcome the cold start difficulties, it is conventional practice to increase the size of hoses connecting the reservoir to the pump inlet and the pump to the steering assist valve in order to enhance flow. Such hoses add costs and require more space in an already crowded environment. Alternatively, one may use a hydraulic fluid, having a viscosity which increases only about 4000 times between 275° F. and −40° F. However, there is a substantial increase in cost over fluid having the usual viscosity properties over the same temperature range. Another remedy involves designing a single passage outlet orifice of a hydro-mechanical flow control device so that it provides higher shear forces to the highly viscous fluid. However, other tradeoffs must be made to the overall operation of the system during normal operating temperatures.
U.S. Pat. No. 5,161,959 proposes various solutions to the problem of cold start noise, including substituting an outlet orifice element having multiple small passages for an outlet orifice element having a single larger passage that is located between the pump outlet of a hydraulic flow control valve and its bypass port. The total wetted surface area of the sidewalls of the multiple small passages is substantially greater than that of the single larger passage. However, by maintaining the same total cross sectional area in the multiple passages as the cross-sectional area of the single larger passage, the pressure drop across the multiple small passages is said to be kept the same as that of the larger passage of the substituted single passage outlet orifice element.